In general, cyan, magenta, and yellow inks, or colorants, cannot produce a high quality black image when combined. To enhance the shadow details of printed images, and to save non-black colorants, a fourth color, black (K), also referred to herein as “key,” is used. However, with the addition of black, many colors may be produced by using multiple sets of C, M, Y and K. The goal, therefore, is to find a way to optimally use C, M, Y and K.
The prior art teaches two primary approaches for achieving this goal. one approach is to use a black generation curve and three under color removal (UCR) curves. An example of such a set of curves is shown generally at 20 in FIG. 1. In the example of FIG. 1, a magenta UCR curve 22 and yellow UCR curve 24 happen to follow the same points. A cyan UCR curve is depicted at 26, while the black generation curve is depicted at 28. These curves are usually implemented as 1-D lookup tables (LUTs), where the input of the curve is used as an index to the LUTs to obtain the outputs of the curves. A desired color, in the red, green, and blue (RGB) color space, to be printed is first converted to initial C, M, and Y colorant amounts using on the following equations, which are somewhat simplified for purposes of illustration:Ci=1.0−R, M1=1.0−G, and Y1=1.0−B,  (1)where RGB and CMY values are normalized to the range between 0.0 and 1.0. The minimum of these initial CMY values, k1=min(C1, Mi, Yi), is used as the index to the 1-D LUT for the black generation curve, BG(k), to obtain the output K amount, as shown in Eq. 2:K=BG(ki).  (2)With the addition of K, the output CMY colorants should be appropriately reduced to produce the desired color. This is accomplished by using the three UCR curves for C, M, and Y, respectively, as shown in Eq. 3:C=Ci−UCRC(k1), M=Mi−UCRM(k1), and Y=Y1−UCRY(k1).  (3)The black generation and UCR curves are experimentally obtained for different printing processes. This basic approach, however, is not flexible enough to fully utilize the printer color gamut resulting from the addition of the K colorant.
Another approach is to use interpolation between the maximum black CMYK and the minimum black CMYK to obtain a desired CMYK. For any given color, i.e., a CIELAB value, there is a range of K values that is possible in the CMYK combination of colorants to produce that color. The lower limit is referred to as Kmin, and the upper limit is referred to a Kmax. A value of K is chosen between these limits, which allows black usage to change throughout the tone scale and color space. A flow chart for this approach is shown in FIG. 2, generally at 30. In this approach, the CMYK printer gamut is divided into a set of CMY gamuts indexed by the K amount. This set of CMY gamuts may be obtained by printing out an extensive set of CMYK patches and then measure them in a device independent color space, such as the CIELAB For a desired color specified in the CIELAB space, all possible CMYK combinations are obtained by 3-D interpolations using this set of CMY gamuts. The final CMYK amounts are obtained by interpolating these CMYK values using a fixed weight, α. This weighting factor controls the desired amount of black to be used. It may be changed depending on the nature of the color to be printed. For example, colors near the neutral axis will have a higher weight for black while more saturated colors will use less black because they will have a lower weight. In most implementations, only CMYK values with the maximum (α=1) and minimum (α=0) K amount are used.
The traditional UCR type of black generation technique is well known in the art. It is discussed in J. A. C. Yule, Principles of Color Reproduction, John Wiley & Sons, New York, 1967. It is also implemented in the PostScript® Language by Adobe Systems Inc., which is described in PostScript® Language Reference Manual, Second Edition, 1990. The newer method of interpolating between CMYK values with maximum and minimum K is discussed in M. Tsukada, et al., New Algorithm for UCR Using Direct Color Mapping, Proceedings of SPIE, Vol. 2413, pp. 365–374, February 1995; and R. Holub et al., The Black Printer, Journal of Imaging Technology, Vol. 15, No. 4, pp. 149–158, August 1989.
U.S. Pat. No. 4,482,917 to Gaulke et al., for Method for a Reproduction of Colored Masters in Four-Color Printing Using Color Reduction, granted Nov. 13, 1984, describes a method wherein the color gamut is divided into a gray and a chromatic region. Different UCR curves are used for the gray and chromatic regions. For colors on the boundary dividing the gray and chromatic regions, CMYK values obtained using both curves are combined to produce a single set of CMYK values for that color.
U.S. Pat. No. 4,551,751, to Jung, for Color Reduction Dependent on Tone, granted Nov. 5, 1985; and U.S. Pat. No. 4,590,515, to Wellendorf for Method and Apparatus for Modifying Color Reduction Depending on Tone, granted May 20, 1986, both describe the use of different UCR curves for image areas with different tone characteristics.
U.S. Pat. No. 5,172,223, to A. Suzuki, et al., for Method of Processing a Color Image to Obtain Color and Black Reproduction Signals, granted Dec. 15, 1992, describes the use of quadratic functions as the UCR curves. In addition to applying the UCR curves, a 3×3 matrix is also used to convert the input CMY values to the output CMY values.
U.S. Pat. No. 5,268,754 to Van de Capelle, et al., for Method and a Device for Converting a Color Coordinate Set, granted Dec. 7, 1993, describes a general strategy to convert signals in an n-dimensional color space to an m-dimensional color space. The case of m=4 is applicable to generating CMYK signals for four-color printers. Rather complex methods involving gradients are described. For methods specific to CMYK printers, only the basic UCR type of techniques is discussed.
U.S. Pat. No. 5,305,119, to R. J. Rolleston et al., for Color Printer Calibration Architecture, granted Apr. 19, 1994, and U.S. Pat. No. 5,528,386, granted Jun. 18, 1996, to R. J. Rolleston, et al., for Color Printer Calibration Architecture, describe a general CMYK printer color table generation process. Black generation using a UCR method is discussed as part of the overall printer calibration system. The traditional UCR method is used.
U.S. Pat. No. 5,402,253 to Seki, for Color Conversion Method and Apparatus with a Variable Gray Component Replacement Ratio, granted Mar. 28, 1995, and U.S. Pat. No. 5,502,579 to Kita, et al., for Color Image Processing Method and Apparatus Capable of Adjusting the UCR Ratio, granted Mar. 26, 1996, describe the use of device independent color values such as the CIELAB and LCH values to modify the black generation and UCR curves.
U.S. Pat. No. 5,425,134 to Ishida, for Print Color Material Amount Determining Method, granted Jun. 13, 1995, describes a method wherein a range of all the CMYK colorants that can produce a desired color is first obtained. A complex algorithm is then used to determine the common range of CMYK values which produce all of the desired colors. The final CMYK colorants for a particular color are selected from the common range.
U.S. Pat. No. 5,508,827 to Po-Chien, for Color Separation Processing Method and Apparatus for a Four Color Printer Using Color Solid Corresponding to Tristimulus Values, granted Apr. 16, 1996, describes the determination of CMYK values with maximum and minimum possible K. Then linear interpolation is used to obtain the final CMYK values based on the desired K amount.
U S. Pat. No. 5,553,199 to Spaulding, et al., for Method and Apparatus for Calibrating a Four Color Printer, granted Sep. 3, 1996, describes the development of a maximum K strategy and a minimum K strategy. The strategies are implemented by 3-D interpolations. For input CMY values, the maximum and minimum K strategies are used to generate two CMYK sets. Then a parameter indicating the desired K amount is used to interpolate these two CMYK sets to obtain the final CMYK values.
U.S. Pat. No. 5,710,824 to Mongeon, for System to Improve Printer Gamut, granted Jan. 20, 1998, describes the use of both the maximum and minimum input CMY values to control the K usage. A non-linear function is used to control the K addition based on the primary colorant amount.